7 SEMICONTINUOUS FUNCTIONS 27 all x e E, then/= sup/n with/n = hn/(l - hn\ and/w is finite and continuous n on E. So we are now in the situation where 0 rg/:g 1 and/has compact support. For each integer n ^ 1 , consider the. decreasing finite sequence of open sets AfaI=:/~1Qfc/H, +00]) (12.7.2), where O^fc^w-1. These sets Akn are n- 1 relatively compact. The function #„ = (l/ri) ]T cpAkn is lower semicontinuous k=l (12.7.5), and we have 0 ^f(x) - gn(x) g 1/fl for all x e E, hence/is the upper envelope of the sequence (#„). On the other hand, since each cpAkn is the upper envelope of a sequence (//jtm«)m£i of continuous functions, it follows that#n is n- 1 the upper envelope of the sequence (hmn)m^ where hmn = OAO E **»•». (2.3.11), k=l and the hmn are continuous. Since gn ^ 1 — (!/«)> tne functions Amn do not take the value 1 ? and it is clear that/is the upper envelope of the " double " family (/uUi.^i (2-3.6). (12.7.9) Let "E be a nonempty metrizable compact space and let f be a lower semicontinuous mapping ofE into R. Then there exists at least one point aeE such that f (a) = inf/(x) (in other words, f attains its lower bound on E). xeE Let ju = inf f(x). Then there exists a decreasing sequence (AM) of elements xeE of R, belonging to/(E) and such that inf /ln = ju. The set/"1^) is therefore the n intersection of the nonempty closed sets Fn =/~1([~~00? ^-J) (12.7.2), which form a decreasing sequence. If their intersection were empty, the open sets Un = E — Fn would cover E. But E is compact and the sequence (Un) is increasing; hence we should have one of the Un equal to E, which is absurd. In particular, if / does not take the value — oo on E, then / is minorized onE. Let (xn) be any sequence of elements of R, and put j;n=infxB+p, zn The sequence (yn) (resp. (zn)) is increasing (resp. decreasing), hence has a limit (4,2.1). We write (12.7.10) lim inf xn =lim/inf xn+p\, lim sup xn =lim /sup xn+p\ «-*-oo M-*OO \JpisO / W-+00 n-*cxD Vp^O / Clearly we have lim sup xn = — lim inf( — xn), so that we need only consider n-*oo /i-*oo the limit inferior.